The present invention is directed to an improved process for the production of aromatic polyester resins.
The aromatic polyester resins employable in applications such as moulding, injection and similar operations need high molecular weights corresponding to values of intrinsic viscosity (IV) generally superior to 0.65-0.75 dl/g. Resins for fibers and film on the contrary have lower IV values, between ca. 0.65 and 0.70 dl/g.
The preparation of the resins is carried out by polycondensation of an aromatic dicarboxylic acid, normally terephthalic acid or its alkyl diesters with an aliphatic glycol at temperature and pressure conditions such as to obtain a resin with final IV values between ca. 0.4 and 0.7 dl/g.
The resin for moulding and similar operations is subjected to polycondensation treatment in the solid state (SSP) with the aim of raising the IV to desired values.
Before the SSP treatment, the resin is subjected to crystallization in conditions such as to obtain a level of crystallinity (35-40%) sufficiently high to allow the conduction of the SSP stage without the inconvenience caused by sticking of particles, which under severe conditions can lead to plant stoppage.
The crystallization treatment is realized in fluid bed reactors or similar equipment, by heating the polymer between ca. 160.degree. C. and 220.degree. C.
The SSP treatment is normally carried out at temperatures between 180.degree. and 220.degree. C. Higher temperatures, which could allow a remarkable increase in the polycondensation kinetic due to particle sticking phenomen caused by inadequate crystallization of the polymer, are not practically employable.
It is known to the inventor that the crystallization speed of polyester resins with relatively high intrinsic viscosity (higher than 0.5% dl/g) is by far much lower than the crystallization speed of resins with low IV (lower than 0.4 dl/g). It is also known to the inventor that by crystallization from the melting of resin at low IV, it is possible to obtain crystallized resins where the DSC thereof curves do not show presence of premelt peaks.